Pollution control laws of various states govern the hydrocarbon emissions from printing equipment or the like, for example where organic materials come into contact with the flame of the burner during the drying process, such materials are limited to a certain pound per hour rate. If the organic solvents used in the ink are of a certain type, the emission rate can be increased provided there is no flame contact with the recirculated air.
Various prior art heating devices have been used for the purpose of raising the temperature of the recirculating air in the dryer. One example is shown in the U.S. Pat. No. 2,224,544 issued Dec. 10, 1940 to Keller and wherein the air to be heated flows enclosed in pipes and which therefore completely separates the air from the main flame and the products of combustion. However, such a device has its shortcomings in that it is expensive to handle the air in this fashion and because of other shortcomings.